The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
A current design class of multiple speed automatic transmissions comprehends a plurality of planetary gear assemblies having their sun gears, planet carriers and ring gears interconnected by permanent connections and selectively connected by clutches or grounded by brakes. The clutched and brakes are generally engaged or activated by hydraulic fluid pressure.
The speed and force with which the clutches and brakes can, should or must be engaged is a major factor in the performance of such transmissions and is the subject of much study. If speed is the more significant design parameter, the clutch piston can be made smaller such that a typical flow of pressurized fluid will quickly move the piston. On the other hand, if force is more important, the clutch piston may be made larger (have a larger area). While it will obviously not move as quickly, given the same pressurized fluid flow provided to a smaller piston, it will be capable of generating higher force.
An acknowledged engineering compromise to this problem is a clutch engaged or activated by a dual area piston. The flow of pressurized hydraulic fluid is provided to a modulating control valve and thence to a shift valve which directs the modulated flow of fluid to the selected larger or smaller area of the piston. Thus, by incorporation and control of the shift valve, a single clutch and piston can produce rapid, low force movement utilizing the smaller area of the piston and slower, higher force movement utilizing the larger area of the piston.
In practice, however, this approach has revealed a shortcoming. In certain shift situations, it is necessary to reposition the shift valve in the middle of a shift. Whenever this is done, the pressure level on one side is immediately transferred to the other. When this occurs during a shift, poor torque capacity control may result.
Thus it is apparent that improvements in dual area clutches and actuating systems for use in automatic transmissions are desirable.